1. Field of the Invention
This invention is directed to elastomer-based insulation for rocket motors, such as the type interposed between a solid propellant grain and a rocket motor casing to protect the casing from high temperatures experienced during burning of the solid propellant grain. In particular, this invention is directed to a solid rocket motor insulation composition that is relatively insensitive to process variables such as moisture contamination and relative humidity, yet upon curing exhibits excellent physical properties and thermal and ablative performances.
2. State of the Art
Solid rocket motors typically include an outer casing or case housing a solid propellant grain. The rocket motor casing is conventionally manufactured from a rigid, yet durable, material such as steel or filament-wound composite. The propellant is housed within the casing and is formulated from a composition designed to undergo combustion while producing the requisite thrust for attaining rocket motor propulsion.
During operation, a heat insulating layer or layers (insulation) protects the rocket motor casing from heat and erosion caused by particle streams generated by combustion of the propellant. Typically, the insulation is bonded to the inner surface of the casing and is generally fabricated from a composition that, upon curing, is capable of withstanding the high temperature gases and erosive particles produced while the propellant grain burns. A liner layer (liner) functions to bond the propellant grain to the insulating layer and to any noninsulated portions of the casing. Liners also have an ablative function, inhibiting burning of the insulation at liner-to-insulation interfaces. Liner compositions are generally known to those skilled in the art. An exemplary liner composition and process for applying the same is disclosed in U.S. Pat. No. 5,767,221, the disclosure of which is incorporated herein by reference.
The combustion of solid rocket propellant generates extreme conditions within the rocket motor casing. For example, temperatures inside the rocket motor casing typically reach 2,760° C. (5,000° F.). These factors combine to create a high degree of turbulence within the rocket motor casing. In addition, the gases produced during propellant combustion typically contain high-energy particles that, under a turbulent environment such as encountered in a rocket motor, can erode the rocket motor insulation. If the propellant penetrates through the insulation and liner, the casing may melt, causing the rocket motor to fail. Thus, it is crucial that insulation withstands the extreme conditions experienced during propellant combustion and protects the casing from the burning propellant. Unless the insulation is capable of withstanding such conditions, failure may occur.
Further, once formulated but prior to full curing, the insulation composition must also possess acceptable shelf life characteristics such that the insulation composition remains sufficiently pliable until used in application to the rocket motor casing. This requirement is essential because the production of a given lot of insulation may have to wait in storage for a number of months prior to cure and installation. Similarly, after application to a rocket motor casing and subsequent curing, a functionally acceptable solid propellant rocket motor insulation must survive aging tests. Rocket motors may be fully fabricated many months before actual firing; in the case of tactical weapons especially, rocket motors may be fabricated as much as a year before actual firing. Over that period of time, the insulation composition must continue to remain fully functional without unacceptable migration of its components to or from adjacent interfacial surfaces and adequately retain its elastomeric characteristics to prevent brittleness. These requirements need to be satisfied under extremely wide temperature variations.
After application of the insulation to the interior of the rocket motor casing, and subsequent to curing thereof, an acceptable cured insulation must also exhibit satisfactory bonding characteristics to a variety of adjacent surfaces. Such surfaces include the internal surface of the rocket motor casing itself. The insulation must also exhibit adequate bonding characteristics with the propellant grain, or with a liner surface interposed between the insulation and propellant grain.
Further, cured insulation must meet the ablation limits for protection of the rocket motor casing throughout the propellant bum without adding undue weight to the motor.
In the past, candidates for making rocket motor insulation have included filled and unfilled rubbers and plastics such as phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins, and the like. The latter plastics, however, crack and/or blister as a result of the rapid temperature and pressure fluctuations experienced during combustion.
Elastomeric candidates have also been investigated and used. The elastomers are used in a large number of rocket motors because their thermal and ablative properties are particularly suited for rocket motor applications. However, the mechanical properties of elastomers, such as elongation capabilities and tensile strength, are often inadequate for rocket motor operation and processing. For example, cured elastomeric insulation, whether thermosetting or thermoplastic, often becomes brittle and cracks in operation unless reinforced with suitable fillers. The cracking of the cured elastomeric insulation creates paths through the insulation which expose the casing to the combustion reaction, thereby rendering the casing more susceptible to failure.
In order to improve the mechanical properties of elastomeric insulation, it has been proposed to reinforce the elastomeric insulation with precipitated silica or silicate. The presence of precipitated silica or silicate in elastomeric rocket motor insulation advantageously improves-the mechanical properties of the elastomer matrix, and further has the secondary benefit of improving the thermal and ablative performance of the insulation. The use of precipitated silica is reported, by way of example, in U.S. Pat. No. 5,498,649 to Guillot. However, because silica and silicate particles are hydrophilic, insulation compositions containing precipitated silica and/or silicate are provided to absorb significant amounts of moisture when exposed to humid environments. High moisture content in a rocket motor insulation can adversely affect bonding characteristics of the insulation, especially at moisture sensitive interfaces, such as the insulation-to-casing bond interface and the insulation-to-liner bond interface. The later bond interface is particularly sensitive to moisture because of the isocyanates typically used in liner formulations.
To address these problems, dry cycles have been implemented to control the moisture content during the manufacture of the insulation and while insulating the rocket motor case. However, the practice of these requisite dry cycles complicates and prolongs processing. Thus, where hydrophilic silica and/or silicate particles are used in insulation compositions, very rigorous process controls commonly are imposed to account for process variables such as moisture contamination and relative humidity.